Hello Nicolas,
It’s possible that you simply need to do a complete uninstallation first, and then reinstall Mailbutler. To uninstall, please see these instructions: https://support.mailbutler.io/knowledge-base/uninstall-mailbutler/#tab-applemail
After that, please try reinstalling Mailbutler from this link: https://bowtie.mailbutler.io/downloads. This link will always provide the most current version of Mailbutler.
If you’re a Mojave or Catalina OS user, please do the following: go to Mail-Preferences-General-Manage Plug-ins and re-enable Mailbutler.
If you’re still receiving this error message, please let me know and tell me which OS you’re using, and if you have any other plugins installed.